As to a liquid crystal projector for enlarged projection of images, an increase in luminance and a decrease in size have been progressing year by year, and a high-output ultra-high pressure mercury lamp generating strong UV rays has come to be used as a light source. Since the optical system has been reduced in size, the energy density of the light transmitted through the optical system has been enhanced. Therefore, the problem that the component parts using an organic material such as a liquid crystal panel, a polarizing plate, a phase difference plate, etc. which are used in the optical system inside the liquid crystal projector are deteriorated principally by the UV rays and further by shorter-wavelength rays among visible rays with the result of a reduction in display quality in a short time has become greater. In addition, there is the problem that the liquid crystal display panel absorbs such light to be raised in temperature through heat generation, leading to generation of non-uniformity of the projected image.
In view of this, in the liquid crystal projector, a UV cut filter is disposed in an optical path between the light source and the liquid crystal display panel, for protecting the liquid crystal display panel and other component parts from UV rays and, further, shorter-wavelength rays among visible rays which are generated from the light source.
As the UV cut filter, a UV absorptive glass capable of absorbing UV rays or a UV reflective glass comprising a glass substrate provided thereon with a UV reflection film capable of reflecting UV rays is used.
The UV absorptive glass uses a glass substrate for absorbing UV rays, and the wavelengths to be absorbed (cut) and the leading edge characteristic (steepness) of the absorption-to-transmission transition can be controlled by selecting the composition of the material and the thickness of the glass substrate.
However, the UV absorptive glass has the problem that the selection of the wavelengths to be absorbed is limited by the material of the glass substrate and the problem that, since the energy of the absorbed light is converted into heat, the glass substrate would be broken due to the temperature rise when strong light is incident on the UV absorptive glass. This danger has been increasing due to the rise in the energy density of the light in recent years. In addition, even with the material selected and regulated carefully, the transmittance at short wavelengths near the wavelengths to be cut cannot be much enhanced, so that attenuation of the light transmitted through the UV absorptive glass is generated.
On the other hand, in the UV reflective glass, the wavelengths to be cut (reflected) can be arbitrarily selected by conditioning the multi-layer film dielectric constituting the UV reflection film, the leading edge characteristic is steep, and the transmittance at short wavelengths near the wavelengths to be cut can be enhanced.
The UV reflective glass is a kind of multi-layer film cut filter called edge filter. The multi-layer film cut filter has a structure in which a multi-layer film dielectric comprised of an alternate lamination of a high-refractive-index layer and a low-refractive-index layer in predetermined optical film thicknesses (=refractive index n×geometric film thickness d) is formed on a light-transmitting substrate by a vacuum vapor deposition method etc., and can cut the light of wavelengths shorter than a specified wavelength and transmit longer-wavelength light.
However, the UV reflection film composed of the multi-layer film dielectric is known to be extremely difficult to produce. Specifically, in order to achieve a steeper leading edge characteristic, the number of times of film formation in alternately forming the high-refractive-index layers and the low-refractive-index layers must be extremely increased; for example, 30 layers or more must be formed. In addition, the film thickness of each layer is small, particularly in the UV region, and the control of the film thickness of each layer must be conducted with high accuracy for the purpose of setting the leading edge wavelength with high accuracy. It is said that the leading edge wavelength is shifted by 5 nm when the film thickness of each layer is shifted by 1%, for example. According to the film formation technology at present, it is difficult to control the film thickness with high accuracy and thereby to form a UV reflection film having the characteristics as designed.
Accordingly, a multi-layer film cut filter permitting control of film thickness with high accuracy and having characteristics as designed and a production method therefor are requested.
Besides, the functions of the UV cut filter in the liquid crystal projector are to completely shut up UV rays with wavelengths of not more than 400 nm and a part of visible rays near the UV rays, to prevent deterioration of organic component parts due to such rays, and to prolong the life of the product. In addition, it is demanded to cut a part of blue light from an ultra-high pressure mercury lamp which emits light excessively rich in blue, and thereby to improve color balance.
Therefore, the UV cut filter for used in the liquid crystal projector is required to have a transmittance characteristic adjusted to the luminance characteristic of the ultra-high pressure mercury lamp. However, it has been difficult to say that the conventional UV cut filter has a transmittance characteristic adjusted to the luminance characteristic of the ultra-high pressure mercury lamp. FIG. 17 shows the luminance characteristic of the ultra-high pressure mercury lamp.
In the luminance characteristic of the ultra-high pressure mercury lamp indicated by the thin broken line in the figure, blue light is excessively much, and the blue light is still strong even if the light at the peak near 405 nm within the blue wavelength range is substantially cut, so that the light at the peak near 440 nm must further be cut by about 10 to 30%.
In FIG. 17, one example of the spectral transmittance of a UV absorptive glass is indicated by the broken line. In addition, one example of the spectral transmittance of a UV reflection film composed of a dielectric multi-layer film is indicated by the solid line.
The spectral transmittance of the UV absorptive glass is such that the peak near 405 nm can be substantially completely cut, but the spectral transmittance is gradual near 440 nm, and it is difficult to enhance the transmittance at short wavelengths near the wavelengths to be cut. Therefore, there is the problem that attenuation of the light transmitted through the UV absorptive glass is generated.
On the other hand, the spectral transmittance of the UV reflection film has a steep leading edge. In order to condition the transmittance at the peak near 440 nm in the UV reflection film, it is necessary to set the half-power point (the wavelength at which a transmittance equal to one half of the maximum transmittance of the filter is shown) at the leading edge in the vicinity of 430 nm. The spectral transmittance of the UV reflection film shown in FIG. 17 has a half-power point of 433 nm.
However, the UV reflection film is composed of a multi-layer film of a dielectric, and is composed, for example, of a multi-layer film having no less than 33 layers. As has been described above, the control of film thickness of each layer must be conducted with high accuracy in order to set the leading edge wavelength with high accuracy, and it is said that the leading edge wavelength is shifted by 5 nm when the film thickness of each layer is shifted by 1%. Under ordinary production conditions, the accuracy of the half-power point, even with best accuracy, is ±4 nm. Therefore, it is extremely difficult to produce a UV reflection film having a half-power point accurately controlled into the vicinity of 430 nm, so that there is the problem that the transmittance at the peak near 440 nm is largely varied due to a difference in half-power point arising from slight scatter of production conditions.
Accordingly, there is a demand for a UV cut filter capable of securely having a transmittance characteristic adjusted to the luminance characteristic of the ultra-high pressure mercury lamp, and a projection type display unit using the UV cut filter.
Furthermore, the liquid crystal projectors have shown a tendency toward reductions in size and cost in recent years, and it has been keenly demanded to reduce the number of component parts. Therefore, there is a request for a projection type display unit which makes it possible to reduce the UV cut filters as component parts.
The present invention has been made in consideration of the above-mentioned requests and demands. Accordingly, it is a first object of the present invention to provide a multi-layer film cut filter permitting control of film thickness with high accuracy and therefore having characteristics as designed.
A second object of the present invention is to provide a method of producing a multi-layer film cut filter by which film thickness can be easily controlled with high accuracy.
A third object of the present invention is to provide a UV cut filter which securely has a transmittance characteristic adjusted to the luminance characteristic of an ultra-high pressure mercury lamp.
A fourth object of the present invention is to provide a projection type display unit using a UV cut filter having a transmittance characteristic adjusted to the luminance characteristic of an ultra-high pressure mercury lamp.
A fifth object of the present invention is to provide a dustproof glass with which UV cut filers as component parts in a projection type display unit can be reduced.
A sixth object of the present invention is to provide a display panel with which UV cut filters as component parts in a projection type display unit can be reduced.
A seventh object of the present invention is to provide a projection type display unit with which UV cut filters as component parts can be reduced.